1. Field of the Disclosure
The present disclosure relates generally to the manufacture of integrated circuits and more particularly to a method of doping integrated circuits.
2. Description of the Related Art
The difference between electron mobility and hole mobility in silicon channel regions of PMOS and NMOS devices, respectively, is a well-recognized issue in the manufacturing of CMOS semiconductor devices. One proposed method of increasing hole mobility in PMOS devices to more closely match the electron mobility of NMOS devices is to form a channel region including SiGe, which is known to facilitate greater hole mobility than silicon without Germanium. This is typically accomplished by forming a strained SiGe epitaxial layer over silicon at PMOS channel regions prior to forming the PMOS transistor gate.
While the use of SiGe channels improves hole mobility in PMOS devices, their use introduces multiple materials under the gate, SiGe and Si, that have different dopant diffusion properties. This difference in diffusion properties complicates formation of source/drain extension regions. For example, diffusion of implanted Boron below the PMOS gate occurs at a faster rate in the Silicon layer than in the SiGe layer. This difference in Boron diffusion rates results in greater lateral diffusion of Boron in the Silicon layer than in the SiGe layer, which produces an extension region in the Silicon portion of the channel that has a source/drain junction spacing that is less than the source/drain junction spacing within the SiGe portion of the channel. The smaller source/drain junction spacing within the Silicon portion of the channel is disadvantageous in that it increases the leakage of PMOS transistors. Therefore, a device and method of overcoming these problems would be useful.
The use of the same reference symbols in different drawings indicates similar or identical items.